High-density Polyethylene (HDPE) is a thermoplastic made from petroleum. It is commonly used to produce plastic bottles, milk jugs, and plastic bags. HDPE has little branching, which gives it its density and strength. HDPE is a #2 plastic recyclable material and is easily recyclable.

HDPE can be used as filament for 3D printers using Fused Deposition Modeling (FDM) technology. The user first creates a 3D model, converts the STL or OBJ file using the printer's interfacing software, which slices the model into sections and determines how to print the layer, and is then sent to the 3D printer for printing. the spooled filament is extruded through a heated nozzle and printed layer by layer until the 3D part is complete.

Joshua Pearce, an associate professor of Michigan Technological University, is working to use recycled milk jugs to turn into filament for 3D printing. Pearce and a team of researchers created a free, open-source recycling unit called RecycleBot. To use RecycleBot, first prepare the milk jugs by washing them and taking off the label, and then shred the jug. Then run the plastic through RecycleBot, which will melt the plastic and convert it to filament. The plans for RecycleBot are available for free at Thingiverse, Thing 54180.